Method of increasing contrast in electrophoretic reproduction



Sept. 8, 1970 w, c, YORK ETAL 3,527,684

METHOD OF INCREASING CONTRAST IN ELECTROPHORETIC REPRODUCTION Filed March 13, 1967' INVENTOR5 J7 W/L 4 x444 6. 709K J'AMFJ 6. d'AkV/S AGENT United States Patent O 3,527,684 METHOD OF INCREASING CONTRAST IN ELECTROPHORETIC REPRODUCTION William C. York and James G. Jarvis, Rochester, N.Y.,

assignors to Eastman Kodak Company, Rochester,

N.Y., a corporation of New Jersey Filed Mar. 13, 1967, Ser. No. 622,499 Int. Cl. 301k /02 US. Cl. 204-181 5 Claims ABSTRACT OF THE DISCLOSURE A method for increasing the contrast between the exposed and unexposed areas of a pattern of conductivity induced in a photoconductive layer, in which said layer is treated prior to exposure so that charged particles will be repelled from the unexposed areas upon development of said pattern.

FIELD OF THE INVENTION This invention relates to electrophotographic reproduction and more particularly to an improved electrophoretic process by which contrast between the exposed and unexposed areas of a photoconductive material is increased upon developing the imagewise pattern by deposition of charged particles.

DESCRIPTION OF PRIOR ART It is known to develop a latent electrostatic image by means of an electrolytic development process in which an aqueous dispersion of developer is applied to a photoconductive material. There are relatively large quantities of charge involved in a process of this type because a ffiOW of at least one one-hundredth coulomb per square centimeter is required to provide a useful density. Further, such flow must occur within about two seconds and implies that a current of at least one one-hundredth ampere per square centimeter is needed. Desirably useful organic photoconductors, such as those based on triphenylamine, would be ineffective in such an aqueous developer system, because the resistance of the photoconductor per unit of area would be much too high to permit obtaining the required current flow with practical voltages. Further, there is deposition of charged particles on the unexposed areas of the photoconductive material for at least some part of the time of current flow which serves to decrease the contrast between the image and background areas.

SUMMARY OF THE INVENTION The invention relates to a system comprising a photoconductive material contiguously associated with an electrically conductive support which constitute a composite record sheet. An electrically insulating organic liquid vehicle is used for the development bath and contains dispersed charged toner particles. The record sheet is positioned in the bath and in spaced relation to a development electrode, a power supply including a switch being connected between the support and the electrode.

The use in this invention of a developer having charged particles in an electrically insulating organic (nonaqueous) liquid provides several advantages: (1) the photoconductive material can be precharged to enhance contrast; (2) a high impedance photoconductor, such as amorphous selenium or any one of many other organic photoconductive compounds, which leads to lower current density, hence, a smaller power supply, can be used; and (3) a negative-positive or a positive-positive can be made.

Considered generally, the invention involves exposing "ice a pattern of radiation on the surface of a photoconductive layer applied to an electrically conductive support or sheet. This structure comprises a record sheet which is immersed in a developer bath with the photoconductive layer substantially parallel to, spaced from and facing a development electrode in the bath. Both the record sheet and the electrode are electrically connected in series with one another through a power supply, Le, a source of electrical potential including a switch, one side of the source being connected to the support and the other side connected to the electrode. As is known in the art, either one of the support or the electrode can be transparent so as to enable the pattern of radiation to be incident on the photoconductive layer. In one arrangement, the pattern of radiation is projected through the bath and through the transparent conductive support to the photoconductive layer.

The developer-treatment bath comprises a dispersion of electrically charged toner or developer particles in an electrically insulating organic liquid which is inert to the charged particles and the electrodes and stable against any change in an electrical field.

One object of the invention is to provide an improved electrophotographic reproduction system for obtaining an imagewise reproduction of improved contrast.

Another object of the present invention is to provide an improved electrophotographic reproduction system in which a photoconductive material is treated either prior or subsequent to exposure for obtaining increased contrast between the exposed and unexposed areas of the imagewise reproduction.

A further object of the invention is to provide an improved electrophotographic reproduction system in which lower current densities, and hence a smaller power supply are required to obtain a reproduction having increased contrast.

Yet another object of the invention is to provide an improved electrophotographic reproduction system in which an electrically insulating organic (nonaqueous) liquid having a dispersion of charged particles is used as a developer of the electrostatic image.

Still another object of the invention is to provide an improved electrophotographic reproduction system in which exposure and development of a photoconductive record sheet can be carried out simultaneously and either a negative-positive or a positive-positive reproduction can be obtained in accordance with the color of the charged particles that are dispersed in the developer liquid.

Other objects and advantages of the invention will be apparent to those skilled in the art by the more detailed description and examples set forth hereinbelow.

DESCRIPTION OF THE DRAWING Reference is now made to the accompanying drawing wherein like reference numerals designate like parts and wherein:

FIG. 1 is a schematic vertical cross-sectional view of one embodiment of the invention in which the record sheet is the anode and the development electrode (i.e., the cathode) is transparent;

FIG. 2 is a schematic view similar to that of FIG. 1 1n which the record sheet comprises a transparent conductive support which serves as the cathode; and

FIG. 3 is a schematic vertical cross-sectional view of another embodiment of the invention in which an imagewise reproduction is obtained by the use of a reusable photoconductive belt.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, a cassette 10 or similar container holds the developer 11 comprising an electrically insulating organic liquid 12 wherein there are dispersed negatively charged toner or developer particles 13 (FIG. 1) or positively charged particles 14 (FIG. 2). The record sheet 15 (FIG. 1), comprising an electrically conductive support 21 and a layer of photoconductive material 22, is supported on and insulated from the bottom of cassette 10, the support 21 being connected by a suitably arranged conductor through a switch 17 to a source of potential 18. The source of potential 18 is such as to be capable of producing an electrical field of about a few thousand volts per centimeter between record sheet 15 and a development electrode 19 which can have a central opening or can be transparent, such as a NESA glass electrode (a product of Pittsburgh Plate Glass Co.) composed of a glass sheet having an electrically conductive coating containing stannous oxide on one surface. The source of potential 18 is also connected through a conductor to electrode 19 which is supported by suitable insulator brackets 20 from the top rim of cassette and thus insulated from it.

In FIG. 2, the record sheet is supported on brackets with the layer of photoconductive material 22 facing electrode 19 which is supported on and insulated from the bottom of cassette 10. In this embodiment, support 21 is transparent as well as conductive and can be of cellulose acetate or a similar material that has coated thereon or incorporated therein a material which will provide the necessary electrically conductive characteristics.

In both embodiments (FIG. 1 and .2), a light source 24 emits light which by means of a suitable lens system 25 is directed through an image-bearing transparency 26. The image radiation pattern is projected by lens 25 through the electrode 19 (FIG. 1) or through the transparent support 21 (FIG. 2) onto the layer 22, thereby exposing the photoconductive layer 22 to a corresponding pattern of radiation.

Simultaneously with the commencement of the exposure, or at a later time that is during the continuance of the conductivity thus generated in photoconductor 22, switch 17 is closed. Thus, the charged toner particles (13 negative in FIG. 1, and 14 positive in FIG. 2) collect on the exposed (illuminated) areas of the photoconductor 22 due to the electrical field generated between the conductive support 21 and the electrode 19. This is especially so when the photoconductor is a very good insulator in the dark.

In this case, any accumulation of charged particles in the unexposed (dark) areas assumes a high enough potential to cause discontinuance of particle flow to these areas. However, such an accumulation of particles in the unexposed areas reduces the contrast between the exposed and unexposed areas. On the other hand, the conductivity of the exposed areas of the photoconductor prevents development of any such potential. Accordingly, toner particles continue to accumulate on the exposed areas so long as the circuit is closed, a supply of charged particles is maintained in the developer, and/or photoconductivity persists.

The loss of contrast in the imagewise reproduction due to accumulation of toner particles on the unexposed (background) areas can be prevented by altering or biasing the photoconductive layer with the use of invisible charge carriers having a color that contrasts with that of the charged toner particles used for forming the imagewise reproduction. By this procedure, reproductions having satisfactory contrast can be obtained even with photoconductors which may not be very good insulators.

An example of an invisible charge carrier is a gaseous ion such as is produced by a corona charger. To use such a charge carrier, the photoconductor is charged uniformly by a corona discharge, for example, as schematically shown in FIG. 3 (precharger 32), before it is placed in contact with developer 11. After the precharged photoconductor is contacted by the developer, exposure thereof to the pattern of radiation renders the exposed areas conductive and in the unexposed areas the potential is sufficiently high and of a polarity so as to repel any charged particles. As a result, the charged particles (13 in FIG. 1 and 14 in FIG. 2) accumulate only in those areas which are exposed to the pattern of radiation.

Another way of accomplishing the same result is to subject the photoconductor to a developer having charged particles of low color, such as colorless or white. After a suflicient number of such particles have accumulated in the unexposed areas so the potential in these areas is sufficient to repel the particles, the record sheet is brought into contact with a second developer containing charged particles of high color, such as black or a similar distinctly contrasting color, or charged particles of such contrasting color are added to the bath and the development of the imagewise reproduction then continues as already described.

The reproduction procedures described thus far are for a negative-to-positive reproduction. A positive-to-positive reproduction procedure can be obtained by interchanging the order of using the two contrasting colored toner particles as described above. A positive-to-positive reproduction can also be obtained by using a dark photoconductor on which the developer will deposit low color particles, e.g. white charged toner particles.

As in the xerographic art, the charged particles can be selected for many reasons other than their absorption of light; for example, wettability, reactivity, luminescence, extent of solubility, or ferromagnetism.

The invention is illustrated by, but not restricted to, the following examples.

EXAMPLE 1 A sheet of aluminum foil and paper laminate coated (on its foil side) with triphenylamine organic photoconductor was placed in a metal cassette and spaced oneeighth inch from a NESA glass electrode. The cell then was filled with an organic liquid developer comprising a dispersion of positively charged toner particles in an odorless paint thinner which is a deodorized highly paraffinic mineral spirits containing minimal amounts of naphthenic and aromatic hydrocarbon materials and having a boiling range of from about 352 to 386 F.

The record sheet support and the electrode were then connected to a power supply and switch, as shown schematically in FIG. 1, and a potential of 300 volts (a field of 945 volts per centimeter) was applied across the NESA glass (positive) and the aluminum foil (negative) support for the photoconductor while the photoconductive layer simultaneously was given an imagewise exposure. This exposure was to the image of a silver negative projected through the dispersion of the toner particles in the paint thinner. The illumination was 225 foot candles without the negative in the projector. A 20 second exposure/processing step yielded a reproduction bearing a visible image of high print density but with a relatively undesirable background.

The triphenylamine organic photoconductive coating on the laminate comprised 3 parts of triphenylamine and 7 parts of Vitel' 101X polyester resin (product of Goodyear Tire and Rubber Co.). This coating contained small amounts of crystal violet and 7,12-dioxo-l3-oxadibenzo- (u,h) fiuorene as sensitizers and maleic acid as a stabilizer.

The positively charged toner used in the liquid developer was prepared from 0.3 part of spirit-soluble Nigrosine (product of General Aniline & Film Corp.), 3 parts of Styron PS-Z polystyrene (product of Dow Chemical Co.), and 0.5 part of Rexobase PW cationic surfactant (product of Emkay Chemical Co.) fused together, cooled and coarsely ground. Further reduction in size was obtained by ball milling in an odorless paint thinner and then diluting this mixture with additional paint thinner to a dispersion suitable for use.

EXAMPLE 2 (Use of lower printing voltage) Substantially complete elimination of background deposit and enhanced sharpness of the imagewise reproduction resulted from reducing the inter-electrode spacing so that lower processing voltages can be used, thereby requiring fewer charged particles to alter the potential in the unexposed areas.

A sheet comprising a triphenylamine organic photoconductor coated on an aluminum foil and paper laminate, as in Example 1, was given a low precharging to about +30 volts and was then placed in apparatus such as shown in FIG. I but wherein the interelectrode spacing was reduced to mils (a field of 2240 volts per centimeter). Using the same liquid developer dispersion as was used in Example 1, the precharged, imagewise exposed laminate then was developed with an impressed voltage of 5 6 volts, yielding a much sharper image produced with negligible background deposit.

The possibility of reusing the photoconductive layer was demonstrated by overlaying the developed image with a sheet of bond paper and transferring the image to the paper by passing the combination under a corona wire.

Precharging enables using higher processing potentials for producing images without background deposits as shown by the following example.

EXAMPLE 3 This example employed an organic photoconductorcoated sheet similar to that of Examplel in its use of triphenylamine, but differing by being a two-layer triphenylamine composition, with one sensitized layer and one nonsensitized layer. This so-called double layer triphenylamine photoconductor was prepared by gelatin-subbing a cellulose acetate support to give it improved adhesion. This was followed by applying an electrically conductive layer of an internal lactone of a copolymer of maleic anhydride and vinyl alcohol which had been made conductive by treatment with sodium hydroxide. Over the aforementioned layer, asensitized layer was applied comprising one part of triphenylamine in 3 parts of Vitel 101X polyester resin, including 2% of a thiapyrylium salt 2,6-bis(p-ethylphenyl)-4-(para-pentyloxyphenyl)thiapyrylium perchlorate, as the sensitizer. Over such a sensitized coating, the second triphenylamine layer was applied which was the same as the first layer but-prepared without a sensitizer.

A two-layer triphenylamine photoconductor as described above was given a positive precharge of 400 volts with a corona wire charger to enable the photoconductor to repel toner from the background areas during develop ment. Following the precharge, the photoconductor was then treated in the same way as was the laminate of Example 1, except that the applied potential was raised to 700 volts. The resulting image was of high density and without undesirable background. The precharging enables using higher processing potentials without provoking undesirable background deposits.

EXAMPLE 4 A two-layer triphenylamine coating, as the photoconductor on a transparent conducting cellulose acetate support (such as used in Example 3), was given a +300 volt corona precharge and placed in the electrophoretic cell (similar to that shown in FIG. 1, but modified as in FIG. 2) with the photoconductive surface spaced oneeighth inch from a blackened metal electrode. The photoconductor then was imagewise exposed through its transparent (cellulose acetate) conducting support while a potential difference of 700 volts was applied between the support and the electrode. The imagewise reproduction was markedly sharper than in prior examples, due to the fact that the pattern of radiation did not have as long a path through the liquid dispersion (see FIG. 2) before reaching the photoconductive layer as in FIG. 1 and, hence, was not as scattered.

EXAMPLE 5 Poor insulator used without and with corona charge A zinc oxide-coated paper was prepared by coating 9. paper support with a suspension of four parts of finely divided zinc oxide in one part of a 50-50 n-butyl-isobutyl methyl methylmethacrylate copolymer (a product of E. I. du Pont de Nemours & Co.) without any sensitizer or other additive. The zinc oxide-coated paper was treated as in Example 1. Since a zinc oxide layer is a relatively poor insulator, unless it has been corona charged, the result was a uniform deposit of the toner particles from the liquid developer over the entire liquid contacted zinc oxide photoconductive layer.

The advantage of applying a corona precharge was demonstrated by giving another sheet of the same zinc oxide-coated paper a negative corona precharge of approximately 300 volts to enable the zinc oxide-coated photoconductor to repel toner from the background areas during development. The cell then was filled with a liquid dispersion of negatively charged toner particles. This dispersion was prepared as follows.

0.6 gram of black organic dye pigment Orasol Black B, a product of Ciba Chemical & Dye Co.), 2.0 grams of polystyrene (Styron PS-Z, a product of Dow Chemical Co.) and 0.4 gram of a 6% cobalt naphthenate solution, were admixed in a compatible organic solvent mixture. This mixture was sprayed into a large volume of the odorless paint thinner of Example 1 in which the sprayed particles are insoluble. The resulting mixture was filtered to separate substantially all of the solvent mixture from the solid particles. The latter were then resuspended in a still larger volume of the same paint thinner to obtain the desired dispersion for use as the toner.

The negatively-charged zinc oxide-coated paper photoconductor then was processed in this dispersion of negatively-charged toner particles in the same way as was the photoconductor in Example 3. The resulting image was of relatively high print density and without undesirable background, similar to the results in Example 3, but of lower maximum density because this negative toner was somewhat inferior to the Nigrosine and polystyrene positive toner used in Example 3.

The Nigrosine black with polystyrene toner particles dispersed in the odorless paint thinner liquid vehicle of the developer dispersion of Example 3 were replaced by corresponding particles of a copper-activated zinc sulfide phosphor (a luminous paint pigment, product of Luminous Products, Inc.) in an amount of 0.5 gram per milliliters of the thinner. A sheet of the same triphenylamine organic photoconductor of Example 3 was then treated in contact with the copper-activated zinc sulfide phosphor containing developer in the same way as in that example. The resulting print manifested high print density, lacked undesirable background, and luminesced in an imagewise pattern when excited with UV radiation.

In any of the specific examples, the photoconductor and/or the charged toner particles can be replaced, respectively, by any other compatible photoconductor or toner particles. In any such substitution of the photoconductor, if the substitute is not a very good insulator in the dark so that the contrast of the reproduction may not be satisfactory, any of the above-described preliminary steps for providing a limited potential, usch as a corona precharge, or preliminarily contacting the photoconductor with charged developer particles having a color, such as white or colorless, can be used to provide the desired contrast. Alternatively, a compatible dark colored photoconductor can be used together with sufficiently light colored to white or colorless charged toner particles in the developer. Also the NESA glass transparent electrode of Example 1 can be replaced by any other similarly suitable transparent electrode.

In the apparatus for contact printing with a reusable photoconductive belt, as schematically shown in FIG. 3, the photoconductor withdrawn by unrolling from a photoconductor supply roll 31 receives a precharge as it is moved over and past a corona precharger 32. The precharged photoconductor is then moved under the transparency 33 and is illuminated by the light from a light source 34 which passes through a variable slit in a mask 35 and, in turn, through transparency 33 to expose the photoconductor.

The black electrode 36 immersed in the developer bath in the cassette motivates'charged toner particles to migrate to the photoconductor in contact with the top of he developer bath upon completing the circuit by closing the switch in the connecting line between power supply 37 and roller contacts engaging the conductive support for the photoconductor.

A transfer paper web fed from a supply roll 38 is brought into contact with the photoconductor transfer rollers 39. The transfer paper with the image reproduction imprinted thereon is then separated from the photoconductor. The photoconductor then passes over a cleaning roller 40 and is taken up for rerun on takeup roll 41. In the case of a projection printing system, the light source 34 and mask 35 can be replaced by an optical system similar to that shown in FIG. 1 with the photoconductor arranged in the focal plane of the optical system.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. A method for producing on a record sheet comprising an electrically conductive support and at least one layer of a photoconductive material on one surface of said support a visible imagewise reproduction of a pattern of radiation having ahigh degree of contrast, the steps which comprise:

charging said layer of photoconductive material uniformly;

positioning said charged record sheet in spaced relation to an electrode in an electrically insulating organic liquid having charged particles dispersed therein which are of one color and of a polarity such that they will accumulate on unexposed areas of said layer, at least one of said support and said electrode being transparent;

exposing said layer of photoconductive material to said pattern of radiation through the one of said support and said electrode that is transparent and while positioned in said liquid for producing a latent electrostatic image of said radiation pattern comprising unexposed and exposed areas on said layer;

connecting a source of potential between said support and said electrode while said sheet is insaid liquid at some time from simultaneously with said exposing step to a time still within the continuance of the conductivity generated in said photoconductive material by said pattern of radiation and for a first time interval suflicient for said particles of one color to accumulate on only the unexposed areas of said layer and until the potential in said unexposed areas is sufficient to repel said particles; and

dispersing a quantity of charged particles, which are of another and contrasting color relative to said particles of one color and of a polarity such that they will accumulate on the exposed areas of said layer, in said liquid after expiration of said first time interval and while continuing said connecting step for an additional time interval sufiicient for said particles of another and contrasting color to accumulate only on the exposed areas of said layer.

2. The method in accordance with claim 1 wherein the liquid having said charged particles of one color dispersed therein is replaced, after said first time interval, by a second electrically insulating organic liquid having said charged particles of another and contrasting color dispersed therein.

3. The method in accordance with claim 1 wherein said photoconductive material is of dark color, the charged particles dispersed in said liquid are of one color and the charged particles dispersed in said liquid after said first time interval are of contrasting color relative to said particles of one color to produce a positive-to-positive reproduction.

4. The method in accordance with claim 1 wherein the charged particles dispersed in said first liquid are of dark color and the charged particles dispersed in said liquid after said first time interval are of light and contrasting color relative to said particles of dark color to produce a positive-to-positive reproduction.

5. The method in accordance with claim 1 wherein said photoconductive material is of dark color, the charged particles dispersed in said liquid are black and the charged particles dispersed in said liquid after said first time interval are white to produce a positive-to-positive reproduction.

References Cited UNITED STATES PATENTS 3,03 8,799 6/ 1962 Metcalfe et al. 3,140,945 7/1964 Metcalfe et al 96-1 3,346,475 10/ 1967 Matkan et al 204181 3,448,030 6/ 1969 Weinberger et a1 96--1 X FOREIGN PATENTS 873,080 7/1961 Great Britain.

GEORGE F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner US. Cl. X.R. 

